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c++: function NTTP argument considered unused [PR53164, PR105848]

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Here at parse time the template argument f (an OVERLOAD) in A<f> gets
resolved ahead of time to the FUNCTION_DECL f<int>, and we defer marking
f<int> as used until instantiation (of g) as usual.

Later when instantiating g the type A<f> (where f has already been
resolved) is non-dependent, so tsubst_aggr_type avoids re-processing its
template arguments, and we end up never actually marking f<int> as used
(which means we never instantiate it) even though A<f>::h() later calls
it, leading to a link error.

This patch works around this issue by looking through ADDR_EXPR when
calling mark_used on the substituted callee of a CALL_EXPR.

	PR c++/53164
	PR c++/105848

gcc/cp/ChangeLog:

	* pt.cc (tsubst_copy_and_build) <case CALL_EXPR>: Look through an
	ADDR_EXPR callee when calling mark_used.

gcc/testsuite/ChangeLog:

	* g++.dg/template/fn-ptr3.C: New test.

(cherry picked from commit 733a792a2b)
This commit is contained in:
Patrick Palka 2022-06-06 14:29:12 -04:00
parent 41487bff13
commit 670ef5b108
2 changed files with 44 additions and 4 deletions
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20
gcc/cp/pt.cc
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@ -20926,10 +20926,22 @@ tsubst_copy_and_build (tree t,
}
/* Remember that there was a reference to this entity. */
if (function != NULL_TREE
&& DECL_P (function)
&& !mark_used (function, complain) && !(complain & tf_error))
RETURN (error_mark_node);
if (function != NULL_TREE)
{
tree inner = function;
if (TREE_CODE (inner) == ADDR_EXPR
&& TREE_CODE (TREE_OPERAND (inner, 0)) == FUNCTION_DECL)
/* We should already have called mark_used when taking the
address of this function, but do so again anyway to make
sure it's odr-used: at worst this is a no-op, but if we
obtained this FUNCTION_DECL as part of ahead-of-time overload
resolution then that call to mark_used wouldn't have marked it
odr-used yet (53164). */
inner = TREE_OPERAND (inner, 0);
if (DECL_P (inner)
&& !mark_used (inner, complain) && !(complain & tf_error))
RETURN (error_mark_node);
}
if (!maybe_fold_fn_template_args (function, complain))
return error_mark_node;

28
gcc/testsuite/g++.dg/template/fn-ptr3.C Normal file
View File

@ -0,0 +1,28 @@
// PR c++/53164
// PR c++/105848
// { dg-do link }
template<class T>
void f(T) { }
template<void (*P)(int)>
struct A {
static void wrap() {
P(0);
}
};
template<void (*P)(char)>
void wrap() {
P(0);
}
template<int>
void g() {
A<f>::wrap();
wrap<f>();
}
int main() {
g<0>();
}